Micromachined planar coil tactile sensor implantable in soft structure

This paper describes design and fabrication of a small, thin and deformable tactile sensing unit including micromachined LC trap circuitry. One tactile sensing element consists of a capacitor and a planar spiral coil. Each element has own resonance frequency, and small deformation of the coil in the element is detected by measuring its frequency deviation. Only three wires are necessary to obtain signals from multiple sensing elements. Since the sensing elements do not need any substrate, they can be implanted in soft material. A prototype array of the sensing elements has been fabricated and tested. Seven-turn coils 1.5 mm in diameter have been obtained. Range of the resonance frequencies of the fabricated elements is 50 MHz - 170 MHz. Using a silicone rubber body in which a sensing element is embedded, push-in depth values are detected by resonance frequency deviations when a wax ball (3 mm in diameter) touches the silicone rubber body. On condition that the sensing element is embedded in standard silicone rubber, its frequency sensitivity is 0.15 %/mm. In the frequency band 50 - 400 MHz, the experiment shows that a simultaneous measurement of not less than 20 elements can be done.